1. Field of the Invention
This invention relates to systems for automatic focus adjustment of optical systems particularly adapted for use in cameras.
2. Description of the Prior Art
Various automatic focusing systems utilizing the non-linearity of photoelectric effect which is priminent particularly in photoconductive cells such as cadmium sulfide and cadmium selenide cells have been proposed for application to apparatus for automatic detection of coincidence between the sharp image and the focal plane in focussing an optical instrument such as camera, or to apparatus for automatic focus adjustment thereof. Examplary of the former apparatus are those described in Japanese Patent Publication Nos. Sho 39-29120 and Sho 41-14500 assigned to the assignee of the present invention, and examplary of the latter is that described in U.S. Pat. No. 3,562,785 to Dwin R. Craig (corresponding to West German Pat. DAS No. 1,173,327 and Japanese Patent Publication No. Sho 44-9501).
The apparatus of Japanese Patent Publication No. Sho 39-29120 utilizes non-linear resistance versus illumination response characteristics inherent to photoconductive cells such as CdS cells for detection of coincidence between the focal plane and the sharp image by sensing the change in resistance of the photoconductive cell produced when the degree of sharpness of the image projected on the photoconductive surface by the optical system is varied.
Japanese Patent Publication No. Sho 41-14500 is an improvement of the apparatus of the above identified Japanese patent wherein a pair of photoconductive cells are arranged in the path of light on the opposite sides of a predetermined focal plane of an optical system and are connected to a differential circuit for providing an output signal in response to a difference between the outputs of the photoconductive cells which is proportional to the degree of focus of the image so that the output signal is indicative of the degree of coincidence between the image and focal plane of the optical system to be focus-adjusted.
U.S. Pat. No. 3,562,785 utilizes fundamentally the principles of Japanese Patent Publication No. Sho 39-29120 and provides an image adjusting method comprising the projection of similar images of an object onto the image-receiving surfaces of two photoconductive cells by means of an optical system, the cells being arranged in such a manner that one of the image-receiving surfaces receives a sharp image at the time that the other receives an un-sharp image, sensing the change in resistance of the photoconductive cell produced by the change in light distribution on the image-receiving surface resulting from the change in the degree of sharpness of the image, and adjusting the optical system until an electrical response signal of the photoconductive cell assembly reaches a maximum value, and apparatus for automatically adjusting the optical system by using the electrical response signal of the photoconductive cell assembly.
The aforesaid non-linear photoelectric effect of a photoconductive cell is a phenomenon that as the degree of sharpness of an image formed on the photoconductive surface is increased, the electrical response and, particularly, the resistance of the photoconductive cell is increased or decreased. This phenomenon is based on the fact that the amount of incident light per an elementary area of the photoconductive surface varies with variation in sharpness of the image and the distribution of such incident light on the network of the elementary areas remarkably differs when the degree of image sharpness reaches a maximum value. In other words, the difference between the light intensities of two adjacent elementary areas is largest when the sharpest image is formed, causing production of a large difference between the resistances of adjacent elementary areas with the photoconductive surface. Most of the objects which may be usually encountered when photographing, however, have very irregular brightness distributions, and further the brightness ratio between two adjacent elementary areas, or the relative contrast in the image is not always sufficiently high. Consequently, with the image-receiving surface of a photoconductive cell of a geometry which has been used in the prior art, it is difficult to accurately detect an optimum sharpness of the image formed thereon owing to insufficient action of the photoconductive effect, and in conclusion, it may be said that the accurate automatic focus adjustment of the optical system is impossible.